1. Technical Field
The technology relates to circuits, systems, and methods for differential conversion of analog to digital signals.
2. Discussion of the Related Art
Analog-to-digital converters (ADCs) are widely used in various electronic apparatus and systems such as mobile phones, audio equipment, image-capture devices, video equipment, wireline communications systems, sensors and measurement equipment, and radar systems, among other applications. A typical ADC is an electronic circuit configured to receive an analog signal, which typically is a time-varying signal, repeatedly sample the analog signal at discrete time intervals, and output a digital signal (e.g., a bit sequence or digital word) for each sampled time interval that is representative of a value of the analog signal during the sampling interval. Because the output of an ADC is an N-bit sequence, the analog signal is discretized into a number M=2N of integer values. The number N is referred to as the bit resolution of the ADC. For example, if a single-ended ADC is an 8-bit device, then an input signal can be discretized into 2N=256 values (0, 1, 2, 3 . . . 255). For an ideal ADC, the output bit value will be linearly proportional to the sampled analog signal value over a full input voltage range that is accepted by the ADC.
There are several types of conventional ADCs, and they may be divided into two groups: ADCs with single-bit quantization (e.g., a comparator) and ADCs with multi-bit quantization (e.g., N-bit ADCs). Multibit quantizing ADCs may include voltage-controlled-oscillator (VCO)-based ADCs, sigma-delta ADCs, successive-approximation-register ADCs, flash ADCs, among others. The multibit, sigma-delta ADCs are finding increasing use in wireless and wireline communication systems. Although these types of ADCs are slower than flash ADCs, for example, they are capable of very high bit resolution and high conversion accuracy. A potential problem with a sigma-delta ADC is that its sigma-delta modulator can introduce spurious noise tones into their output due to nonlinear limit-cycle oscillations inside the modulator loop. These noise tones can lead to conversion errors.